在AppDelegate中使用未声明的标识符？

Question

在我的AppDelegate实现文件中，我收到一个未声明的标识符的错误，但它似乎被声明，我在忽略什么？

#import "AppDelegate.h"
#import "ViewController.h"

@implementation AppDelegate

@synthesize window = _window;
@synthesize managedObjectContext=__managedObjectContext;
@synthesize managedObjectModel=__managedObjectModel;
@synthesize persistentStoreCoordinator=__persistentStoreCoordinator;


- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
// Override point for customization after application launch.
self.viewController = [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
self.window.rootViewController = self.viewController;
[self.window makeKeyAndVisible];
return YES;

- (NSManagedObjectContext *)managedObjectContext

错误发生在此行“使用未声明的标识符”managedObjectContext“

在我的AppDelegate头文件中：

 #import <UIKit/UIKit.h>
 #import <CoreData/CoreData.h>

 @class ViewController;
 @interface AppDelegate : UIResponder <UIApplicationDelegate>

 @property (strong, nonatomic) UIWindow *window;

 @property (strong, nonatomic) ViewController *viewController;

 @property (nonatomic, retain, readonly) NSManagedObjectContext *managedObjectContext;
 @property (nonatomic, strong) NSManagedObjectModel *managedObjectModel;
 @property (nonatomic, strong) NSPersistentStoreCoordinator *persistentStoreCoordinator;

- (NSURL *)applicationDocumentsDirectory;
- (void)saveContext;

@end